Steel filaments and steel cords adapted for the reinforcement of elastomer products such as tires, impact beams, hoses, flexible pipes, . . . are well known in the prior art.
Steel filaments and steel cords are made starting from steel wire rod. This steel wire rod typically has a steel composition along following lines. A carbon content of more than 0.60 per cent by weight, a manganese content ranging between 0.40 per cent and 0.70 per cent by weight, a silicon content ranging between 0.15 per cent and 0.30 per cent by weight, a maximum sulphur and a maximum phosphorus content of 0.03 per cent by weight. Other micro-alloying elements may be added. An example is chromium. The steel wire rod usually has a diameter dS of 5.5 mm or of 6.5 mm.
The wire rod is firstly cleaned by mechanical descaling and/or by chemical pickling in a H2SO4 or HCl solution in order to remove the oxides present on the surface. The wire rod is then rinsed in water and is dried. The dried wire rod is then subjected to a first series of dry drawing operations in order to reduce the diameter until a first intermediate diameter.
At this first intermediate diameter d1, e.g. at about 3.0 to 3.5 mm, the dry drawn steel wire is subjected to a first intermediate heat treatment, called patenting. Patenting means first austenitizing until a temperature of about 1000° C. followed by a transformation phase from austenite to pearlite at a temperature of about 600-650° C. The steel wire is then ready for further mechanical deformation.
Thereafter the steel wire is further dry drawn from the first intermediate diameter d1 until a second intermediate diameter d2 in a second number of diameter reduction steps. The second diameter d2 typically ranges from 1.0 mm to 2.5 mm.
At this second intermediate diameter d2, the steel wire is subjected to a second patenting treatment, i.e. austenitizing again at a temperature of about 1000° C. and thereafter quenching at a temperature of 600 to 650° C. to allow for transformation to pearlite.
If the total reduction in the first and 2nd dry drawing step is not too big a direct drawing operation can be done from wire rod till diameter d2.
After this second patenting treatment the steel wire is usually provided with a brass coating: copper is plated on the steel wire and zinc is plated on the copper. A thermo diffusion treatment is applied to form the brass coating.
The brass-coated steel wire is then subjected to a final series of cross-section reductions by means of wet drawing machines. The final product is a high-tensile steel filament with a carbon content above 0.60 per cent by weight, with a tensile strength above 2000 MPa and adapted for the reinforcement of elastomer products.
Despite its wide-spread use, the above described process has a disadvantage it that it consumes a lot of energy. More particularly, the double patenting process steps and their related austenitizing furnaces require a lot of energy. As a matter of example only, a single austenitizing furnace produces a power of 374 KWatt/Ton of produced steel cord. Indeed the furnaces and the associated quenching process represent a considerable part of the CO2 production during the manufacturing of steel filaments and steel cords adapted for the reinforcement of elastomer products. The patenting process, however, is needed and cannot be cancelled as such. This patenting process restores the metal structure of the steel wire into a state which allows for further drawing. Without this patenting process the steel wires would break frequently during further drawing and would become too brittle.